Modelling and microfabrication of a polyimide-based 3D tissue coupled electrodes for electrical stimulation in the chick retina

We present a new flexible microfabricated polyimide-based platinum array of 3D “tissue coupled electrodes” (TCEs) applied for local stimulation of the chick retinal photoreceptor layer and simultaneous patch clamp recording from a single retinal ganglion cell (RGC) in in vitro conditions. The design is based on a simplified version of the technology created by Palanker et. al. [1]. Stimulating the photoreceptor layer using planar electrodes, we determined threshold currents of activation for the RGCs. The spatial selectivity of neuronal activation by electrode stimulation and repetitive stimulation responses of the RGCs were also studied as a third part of the experiments. The present threshold currents for electrical stimulation of retinal tissue are too high [2] to ensure safe and efficient chronic stimulation. As has been quoted before in [1], this limitation is probably due to poor electrode-tissue interfacial contact. To counter the interfacial problem a chip comprising of four electrodes of four different types that includes previously used planar electrodes was microfabricated. There are 25μm perfusion holes near the electrodes permitting the tissue nutrients and fluid exchange. Simplified model simulation with 100μA stimulation of a ring electrode bearing the surface of a Φ75μm electrode shows localised electric field (voltage gradient) having a value 9000V/m measured at 300-350μm above the stimulating electrode surface. We conclude by stating that we can use lower values of current and still prove that the threshold gradient remains stable around 3000V/m which is the threshold criterion for effective stimulation.